I'm planning to build a dually with an extended cab. I've not got all the details sorted out yet, but my current thought is to start with a suburban body and graft the back of a conventional cab on behind the first set of pasenger windows on the suburban.
I have a parts '52 GMC Suburban that may become the donor for this project, and I have the back half of a '47 5-window cab, and a complete '55.1 cab from which to scavenge the back of the cab.
So far this project is still in the thinking and parts collecting stage, and I'm very interested in seeing what others might be thinking as ways to approach similar problems.